I've had similar purse problems when I've made them with similar straps. You have to be working with very stiff fabric and tons of interfacing to really make that work right. If it is possible to still get inbetween your layers without taking too much apart, I would suggest either iron on another layer of interfacing, or could you maybe hide some plastic canvas in the middle? Hope that helps! Its still really cute even with a saggy middle (as are we all!)

I have a window seat cushion that I would like to cover with something quilted, but not sure which pattern might look nice...which would you prefer to sit on? I am open to suggestions. :) I like that you share your bag creations! I understand that putting a rigid plastic form on the bottom of flat bottomed bags is supposed to prevent sagging.